Cinnabar and SC16 high-pressure phases of ZnSe: An ab initio study

Total-energy calculations of zinc selenide, done at four different phases under conditions of hydrostatic pressure using the all-electron augmented plane-waves method, reveal the stability interval of two intermediate phases, cinnabar and SC16, between the ambient-pressure zinc-blende and high-pressure rocksalt ones. The results of previous calculations are critically discussed and the importance of the full relaxation of all structure parameters demonstrated. Our simulation sheds light on the fact that the cinnabar phase, apparently of higher enthalpy than the (not yet detected) SC16 one, seems to have been definitely identified in high-pressure experiments in the downstroke only.

Figure 1

(Color online) Total energy vs volume for ZnSe in different phases, as calculated within the LDA (left panel) and GGA (right panel). Continuous lines show fit to the Murnaghan equation of state for each phase. In cinnabar and SC16 phases, the internal coordinates have been relaxed in each point.

Figure 2

(Color online) Static enthalpy per atom in different phases of ZnSe, relative to the B3 phase, as calculated with the LDA (left panel) and GGA (right panel).

Figure 3

Scheme of distorted tetrahedric coordinations around anion and cation sites in the cinnabar structure, in the projection onto the $\left(x,y\right)$ plane. Numbers in the circles indicate $z$ coordinates of corresponding atoms.

Figure 4

(Color online) Internal coordinates in the cinnabar phase of ZnSe as function of pressure. Note the collapse into the rocksalt phase at 18 GPa. The crystal structure (around an anion site, as in the left panel of Fig. 3) corresponding to this situation is shown in the inset.

Figure 5

(Color online) Variation in $a$, $c$ lattice parameters in the cinnabar phase of ZnSe as function of pressure. The ideal $c/a=\sqrt{6}$ value indicated is for the rocksalt structure represented as hexagonal one.

Figure 6

Band structure and total density of states as calculated in the cinnabar phase of ZnSe within the GGA at the pressure value corresponding to its presumed stability.

Figure 7

(Color online) Bond lengths and bond angles as calculated under different pressures in the cinnabar structure of ZnSe. See text for details.

Figure 8

(Color online) A side view of the cubic primitive cell of the SC16 structure underlying its trigonal symmetry and indicating the definition of internal coordinates $u$ (for cations) and $v$ (for anions).

Figure 9

Internal coordinates $u$, $v$ (left panels) and lattice parameter (right panel) in the SC16 phase of ZnSe as optimized at different applied pressures.

Figure 10

(Color online) Bond lengths and bond angles in the SC16 phase of ZnSe optimized at different applied pressures. See text for details.